This invention relates to control valves and, more particularly, to a new and highly advantageous precision control valve for providing precise, selective control of fluid flow through the valve, while additionally providing greatly improved response, significantly increased fluid flow rates, and dramatically reduced overall size and weight.
Instrumented control valves, also called final control elements, regulate the flow or pressure of a fluid which affects many controlled process. Control valves are usually operated by remote signals from independent devices using control mechanisms, and may be powered electrically, pneumatically, electro-hydraulically, etc. Control valves are extensively used in many industries, including chemical, petrochemical, oil, water, and gas. Many companies in all industries are having major problems in the areas of energy consumption and wasted product, due to the complexities of their process control systems.
Industry research has revealed that of all the components and instrumentation used in process control systems, it is the control valve that has resulted in a major portion of energy consumed. Moreover, the slow or erratic response of conventional control valves are directly responsible for most of the wasted product. This is reflected in the responses from systems engineers that most plants cannot process a consistent quality product, without them having to constantly adjust the system. The unfortunate result is that after spending a great deal of time setting up a controlled process, the systems engineer out of frustration, may adjust the valve to a compromised setting or operate it manually, resulting in millions of dollars of low quality or wasted product.
The mechanics of control valve design have not changed much over the years. New electronic devices controlled by computers have generally been unable to improve response times of control valves to stabilize controlled systems. Although valves with rapid response times have been used in the aerospace industry, i.e., to shuttle fluids between the wings, they are incapable of regulating, and are used only for on/off or as relief valves. Often the electronic accessories have only made the process system control worse because of their high speed feedback. Such rapid feedback is normally desirable. Constraints in existing control valves cause them to be much too slow to react to the rapid process signal. Moreover, in this situation, the control valve continues to receive additional signals before it can respond to the initial signal, causing the control valve to become erratic and unstable.
A conventional control valve, is typically a globe valve due to its ability to more precisely control flow, uses a totally external driving means, also referred to as control components, to actuate the valve member that is exposed to and effects control of the fluid in the pipeline. This external driving means generally consists of a cumbersome, elongated structure usually much larger than the valve itself, containing a linkage to the control member that is in communication with the fluid in the pipeline, also referred to as the working fluid. This linkage generally includes a shaft and a spring for urging the valve member toward a valve opening position. A large diaphragm, generally located at the top of the elongated structure and supplied by an external pneumatic source, urges the linkage toward a closed position. The design of this external driving means has remained basically unchanged for many years. A conventional control valve generally works as follows: after receiving a signal indicating that the control member needs to actuate to a different position, the pneumatic source adjusts the pressure in a chamber above the diaphragm. The speed of pneumatic line flow to effect this change is limited to the speed of sound. Additionally, the force generated by the diaphragm to effect this change in position of the valve member must first overcome the resisting forces acting on the valve member by the fluid in the pipeline, the inertia in the linkage itself (including, but not limited to the shaft and friction associated with O-rings and packings installed to prevent valve leakage; the term xe2x80x9cstrictionxe2x80x9d has been coined and used by those skilled in the art to refer to any type of mechanical valve restriction), and the spring force (if the valve member is to be moved toward a valve closing position).
Moreover, after sufficient pressure is built up in the chamber above the diaphragm to effect valve member movement, unless the target position is to completely open or to completely close the valve, the linkage inevitably drives the valve member past the target position, commonly referred to as overshoot or gain. The valve member must then be repositioned in the opposite direction, with the overshoot in this direction commonly referred to as droop. This combination of gain and droop is referred to as dead band, which may cause hysteresis. Hysteresis is the tendency of the valve to give a different output for a given input, depending on whether the input resulted from an increase or decrease from the previous value. Hysteresis is distinguished from dead band in that some reversal of output may be expected for any small reversal of input. To then correct gain or droop, the pressure above the diaphragm must again be adjusted, again with the same pneumatic line flow limitations, and again the inertia of the linkage must be overcome. Valve hysteresis must also be taken into consideration. Additionally, a change in pressure may affect the working fluid flow rate, especially if the fluid is a gas, due to compressibility, which is the fractional change in volume of a fluid per unit pressure change. To make matters worse, if the driving means is not mounted in a substantially vertical position, which generally requires that the control valve be installed in a horizontal position, unbalanced forces are introduced into the system due to side loads caused by gravity. These unbalanced forces produce additional friction forces, which further worsen an already excessive response time. Worse yet, even properly installed control valves may experience other adverse effects due to circumstances requiring component re-routing, e.g., clearance problems with an existing plumbing system. This may be due to the installation of geared linkages necessary to achieve the desired vertical position, which produces its own backlash.
A goal of system engineers is to design a control system so that when it is disturbed, the controlled process variable will come under control again as quickly as possible. The time required for a control system to regain control after it has been upset and temporarily goes off control is commonly referred to as recovery time. The time delay between two related events is commonly referred to as dead time. An example of dead time as it relates to the control valve is the delay encountered between the time the signal is sent to effect valve member movement, and the time actual valve member movement is effected. Generally, the recovery time of a control system will increase in direct proportion to the dead time. That is to say, if the dead time is doubled, the control system will take twice as long to stabilize, or regain control. Dead time is the antithesis of effective process control. A 7-8% dead time has been associated with a conventional control valve. A recently conducted study of an industrial control process using such a valve revealed that costs associated with waste could approach US$750,000 annually from just a 2-inch valve above. Moreover, based on flow area (xcfx80r2), waste from an 8-inch valve could approach 16 times that of a 2-inch valve, or US$12,000,000 annually.
In addition to process control problems, both the valves, including the valve housing and control components, as well as downstream components are subjected to fluid flow pulsation surges. These pulsation surges, which invariably occur during any control process, create fluid stresses that act on the valves and downstream components, thereby significantly reducing their service life.
To summarize, conventional control valves are often too slow to respond to industry""s process control needs, costing billions of dollars annually. There is an urgent need for an improved valve design capable of more rapid response. More specifically, a valve capable of effective integration with computerized control is highly desired.
There is also an urgent need for a valve design that weighs significantly less, reduces significantly the size and weight of control components external to the valve housing, handles significantly greater flow rates thereby reducing energy costs associated with control system operations, and is substantially more flexible in application, e.g., can be installed in any attitude, horizontal, vertical, sloped, etc., without degraded performance.
There is also a need for a valve design that dramatically increases the service life of the valve housing. More specifically, an improved valve design is needed that creates boundary layers in the fluid flow to help protect valve components, while simultaneously providing smoother, more uniform flow.
There is also a need for a valve housing design that is capable of absorbing fluid pulsation surges, to help effect more focused, uniform flow of the fluid through the pipeline, which greatly increases the life of downstream components, due to decreased fluid-induced stresses.
There is also a need for a valve design containing a minimum number of moving parts for additional reliability.
There is also a need for a valve having dramatically increased rangeability, which is defined as the ratio of maximum to minimum flow within which all flow characteristics are maintained within prescribed limits.
Accordingly, among the several objects, features and advantages of the invention may be noted the provision of a valve that uses the working fluid in the pipeline to both help balance and to actuate the valve control members. By helping balance the valve members, the size of the actuator required to effect controlled valve member movement is also greatly reduced. In an alternate embodiment, the valve can also use pressurized fluid from an external source to both help balance and to actuate valve members. The valve also has rapid response capabilities and can be configured for efficient computer control. The valve housing in its preferred form, an in-line co-axial configuration, yields significantly increased flow capacity over a globe valve of similar size, the generally utilized valve configuration for control valves.
A coaxial control valve of the present invention may have the flow rate of a ball valve, yet providing the degree of flow control associated with a globe valve. Moreover, the present valve enables the user to control flows down to extremely low levels, e.g., 1.0 cubic centimeter per minute, while reaching flow rates of more than 200 gallons per minute (for a 2-inch valve design of the present invention). This represents a rangeability (see previously provided definition) having an order of magnitude of 105. Rangeability for conventional valves is normally less than 250.
The new coaxial control valves are extremely light and compact. A 2-inch valve of the present invention weighs approximately 35 pounds. Conventional control valves in industry generally weigh from 125 to 170 pounds.
Optionally, a 2-inch valve design of the present invention, by permitting flows of more than 200 gallons per minute, exceeds the flow rates permitted by conventional 2-inch globe valves by a factor of 5, based on flow coefficient values, denoted as Cv, and defined as the flow in gallons per minute of water at 60xc2x0 F. with a pressure drop of one pound per square inch (psi). The Cv value associated with a 2-inch valve of the present invention is more than 200, while the Cv value associated with a 2-inch globe valve is 44.34. By comparison, the Cv value for a 2-inch standard port ball valve is 120, which in its fully open position presents a totally unrestricted cross sectional area through the valve. By permitting dramatically increased flows while using the same amounts of energy, the valve of the present invention also substantially reduces energy costs.
Additionally, by reconfiguring valve components, boundary layers are created in the fluid flow to help protect valve components from frictional wear, while simultaneously providing smoother, more uniform flow.
Additionally, by incorporating pulsation dampening capabilities into the inner workings of the valve, pressure spikes are significantly reduced, thereby reducing fluid-induced stresses and significantly increasing the service life of downstream components. Therefore, although initially intended for use as a control valve, the valve of the present invention may also be configured for at least the following additional uses: shut-off valve, check valve, flow meter, safety valve, relief valve, safety relief valve, velocity control valve, pressure control valve, pulsation dampener and spike attenuator valve and temperature control valve. Other objects and features will be apparent or are pointed out more particularly hereinbelow.
Briefly, a flow control valve of the present invention comprises a housing having a fluid inlet at one fitting end and fluid outlet at another fitting end, and a control body supported within the housing by support structure bridging between the control body and the housing. The housing and control body together define a flow passage communicating with the fluid inlet and outlet for flow of fluid through the valve from the inlet to the outlet along fluid paths symmetric with respect to an axis of the control body. The control body and support structure are streamlined so as not to interfere substantially with the flow of fluid along the flow paths. At least one valve member is carried by the control body in an orientation for being shifted by driven movement along the axis relatively toward and away from the fitting end for control of fluid passing through the fitting end. An actuator within the control body drives the valve member along the axis, and at least one fluid port extends through the support structure for communicating with the actuator and provide for control of the actuator.